As is well known to those skilled in the art, non-volatile memory devices are characterized in that there is no loss of data stored in their memory cells, even when an external power supply is removed. For that reason, such non-volatile memory devices are widely employed in computers, mobile communication systems, memory cards and the like.
Flash memory structures are widely used in such non-volatile memory applications. The typical flash memory device employs memory cells having a stacked gate structure. The stacked gate structure typically includes a tunnel oxide layer, a floating gate, an inter-gate dielectric layer and a control gate electrode, which are sequentially stacked above a channel region. While flash memory structures have enjoyed enormous success, the continued and ever-present drive to reduce the size of integrated circuit products has created many challenges for the continued scaling of flash memory devices. Such challenges include scaling of program/erase voltages, access speed, reliability, the number of charges stored per floating gate, etc.
The limitations of flash memories such as decreased endurance and reliability have sped up the search for alternative non-volatile memories. These limitations become more significant when memory devices scale down in sizes to keep up with the rising demands of the digital world. In recent years, alternative non-volatile memory devices such as resistive random access memory (RRAM) devices have emerged as one of the potential candidates to replace flash memories, owing to simple metal-insulator-metal (MIM) structure, complementary metal oxide semiconductor (CMOS) compatibility, fast access speeds and low power consumptions.
A resistance random access memory (RRAM) device is a simple two-terminal device memory device including two spaced-apart electrodes with a variable resistance material layer or ion conductor layer positioned between the two electrodes. The variable resistance material layer typically includes various metal oxides, such as hafnium oxide, tantalum oxide, nickel oxide, titanium oxide, zirconium oxide, copper oxide, or aluminum oxide. The variable resistance material layer is used as a data storage layer. The resistance of the variable resistance material layer may be varied or changed based upon the polarity and/or amplitude of an applied electric pulse. The electric voltage or electric current density from the pulse, or pulses, is sufficient to switch the physical state of the materials so as to modify the conducting properties of the material and establish a highly localized conductive filament (CF) in the variable resistance material. The pulse is of low enough energy so as not to destroy, or significantly damage, the material. Multiple pulses may be applied to the material to produce incremental changes in properties of the material. One of the properties that can be changed is the resistance of the material. The change may be at least partially reversible using pulses of opposite polarity or pulses having a different amplitude from those used to induce the initial change.
In general, after an RRAM device is initially fabricated, the variable resistance material layer does not exhibit any switching properties. Rather, a so-called FORMING process, a high-voltage, high-current process, is performed to initially form the localized conductive filament with oxygen vacancies from the cathode, establishing a low-resistance state (LRS) exhibiting a relatively high current flow. A so-called RESET process is performed to break the conductive filament and establish a high-resistance state (HRS) exhibiting a relatively low current flow. This RESET process is typically a current-driven thermal process that causes the conductive filament to be broken by a heat-assisted chemical reaction.
The most common switching mechanisms in RRAMs are of valence change memory (VCM) type and electrochemical metallization (ECM) memory type. The switching mechanism of an RRAM is determined by the type of filament formed between the active and inert electrodes of the RRAM. While filaments in VCM are formed by anionic oxygen vacancies, filaments in ECM are formed by cationic species generated from the active electrode; hence redox chemical reactions are of importance in understanding the formation of filaments. The binary memory states of RRAM devices arise from its high resistance state (HRS) and low resistance state (LRS). HRS and LRS occur when the filaments are ruptured and formed between the active and inert electrodes of the RRAM. The applied voltage difference between the active and inert electrodes results in either the formation or rupture of filaments. A SET state occurs when HRS becomes LRS and a RESET state occurs when LRS become HRS. In the SET state, a compliance current limit is imposed to protect the resistive switching devices from high current damages.
Note that the RESET process removes only a portion of the entire length of the conductive filament, i.e., the RESET process does not remove the entire conductive filament. After a RESET process is performed, a so-called SET process is performed to reestablish the conductive filament and thus the low-resistance state of the RRAM device. The SET process is essentially the same as the FORMING process except that the SET process is performed at a lower voltage than the FORMING process since the filament length to be reestablished is shorter than the length of the conductive filament that was formed during the FORMING process. The FORMING process requires a larger voltage than the SET process because the oxide lattice needs to be broken to form a pathway for the filament to grow onto. After the pathway is established, a lower voltage (or energy) is sufficient for the filament to grow on.
One problem associated with typical RRAM device fabrication is that the variable resistance material layer, i.e., hafnium oxide, tantalum oxide, nickel oxide, titanium oxide, zirconium oxide, copper oxide, or aluminum oxide layer, typically must be deposited by atomic layer deposition (ALD), which can be an expensive process. Further, typical RRAM device fabrication is a time consuming process. Also, the filaments established in the variable resistance material can be difficult to study and observe. As a result, there is presently insufficient understanding of the physics of current RRAM operation.
Accordingly, it is desirable to provide an improved NVM device, such as a RRAM device, and improved method for fabricating such devices. It is further desirable to provide RRAM devices with magnesium oxide insulator layers that may be deposited by physical vapor deposition (PVD) or sputtering. It is also desirable to provide methods for fabricating NVM devices that are less expensive and less time consuming than current methods. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.